You're welcome to work on the site using a temporary development server before moving it onto a live/production server. Once the site is live, just be sure to remove the theme from the development server. If the item requires you to register the license/URL, you should first check with the author to see if this can be reset.
If this is simply a URL or domain name switch, and the end product is still the same, then you should contact the author to see if they can reset your license. However, if this is a new end product, you will be required to purchase another license for this product, as these themes are single-use licenses.
You can contact the author directly via their profile page or through the item's support and comments tabs. For more information on where to find their links, you may find this article helpful.